At present, uniaxial pressing is the most widely used tile forming technique in processing ceramic materials. Uniaxial pressing consists of compacting the pressing powder contained in a rigid cavity, by applying pressure in a single axial direction with one or more rigid punches. The rigid cavity is made up of a base, which is usually mobile and known as the bottom punch, and walls, which can be mobile or fixed and are known as the pressing mould die.
The various pressing techniques differ in the movement of the basic mould elements: top punch, bottom punch and die, and in the number of pressure-applying elements.
In single action uniaxial pressing (Figure 1) pressure is applied through the top punch, which enters the cavity holding the pressing powder, consisting of the lower punch and die which remain immobile in this step. After compacting the piece, the top punch withdraws and the movement of the lower punch ejects the piece from the mould. As the friction between the powder particles, and between particles and mould walls causes density gradients in the piece, single action uniaxial pressing is only used for producing pieces with a simple geometry having reduced thickness.
Double action uniaxial pressing (Figure 2) is used when the piece is too thick for the single action technique. In these cases, both the top as well as the bottom punch apply pressure to the pressing powder contained in the die. Once load application has ended, the top punch is withdrawn and the upward movement of the lower punch ejects the piece. In this case density distribution in the piece is symmetric, as shown in Figure 3.
In the cases in which friction between the pressing powder and the walls of the mould leads to excessive inconsistency in body density and/or cause crack formation on ejecting the piece, it is advisable for the die also to be mobile. In this technique, during the compaction step, the die partly accompanies the movement of the top punch, and in the ejection step the die descends while the top punch rises, to facilitate extraction of the piece. This technique is known as (single or double action) uniaxial pressing with a floating mould or die (Figure 4).